Display panel comprising a light guide plate

ABSTRACT

The display panel ( 21 ) comprises a light guide ( 2 ), a second plate ( 4 ) and a movable element ( 3 ) between both plates for locally decoupling light out of the light guide ( 2 ). Voltages, applied to electrodes ( 5,6  resp.  25 ), associated with the light guide ( 2 ), the second plate ( 4 ) and the movable element ( 3 ), locally bring the movable element ( 3 ) into 5 contact with the light guide plate ( 2 ) or the second plate ( 4 ). If, between the light guide ( 2 ) and the first electrodes ( 5 ) a layer ( 51 ) is present with a second refractive index being smaller than the first refractive index of the light guide ( 2 ), and the light guide ( 2 ) is positioned between the first electrodes ( 5 ) and the movable element ( 3 ), absorption of the light transported through the light guide plate ( 2 ) is reduced, providing a display panel ( 21 ) with 10 improved power efficiency.

The invention relates to a display panel comprising

-   -   a light guide, having a first refractive index,    -   a second plate facing the light guide,    -   first electrodes and second electrodes, associated with the        light guide and the second plate, respectively, and    -   a movable element provided with a third electrode, between the        light guide and the second plate, for locally decoupling light        out of the light guide,    -   the electrodes being present for locally bringing the movable        element into contact with the light guide by the application of        voltages to the electrodes.

An embodiment of the display panel of the type mentioned in the openingparagraph is known from WO 99/28890.

The known display panel comprises a light guide in the form of a lightguide plate, in which, in operation, light, generated by a light source,is trapped so that this plate forms a light guide. First and secondelectrodes are positioned at the surface of the light guide plate facingthe movable element and at the surface of the second plate facing themovable element, respectively. By the application of voltages to thefirst and second electrodes and the third electrode, the movable elementis locally brought into contact with or set free from the light guideplate. Insulating layers are present on the surface of the first andsecond electrodes facing the movable element to prevent directelectrical contact between the first and second electrodes on the onehand and the third electrode on the other hand.

A drawback of the known display panel is that a relatively small portionof the light coupled into the light guide plate is available for beinglocally decoupled out of the light guide plate because a relativelylarge portion of the light coupled into the light guide plate isabsorbed during transport through the light guide plate. Therefore theknown panel requires, in operation, relatively much energy to operate alight source producing a relatively large amount of light.

It is an object of the invention to provide a display panel of the kindmentioned in the opening paragraph, having an improved power efficiency,i.e. the same amount of light is available for being locally decoupledout of the light guide at a reduced power. The object is therebyachieved in that between the light guide and the first electrodes alayer is present with a second refractive index being smaller than thefirst refractive index, and the light guide is positioned between thefirst electrodes and the movable element.

The invention is based on the insight that the absorption of light bythe first electrodes, during transport of light through the light guide,is reduced if only a portion of the light guided by the light guidereaches the first electrodes. A portion of the light is alreadyreflected before reaching the first electrodes, if a layer is introducedwith a second refractive index smaller than the first refractive index.This is in contrast to the display panel known from WO 99/28890 wherethe light guided by the light guide plate reaches the first electrodesbecause the first electrodes are positioned on the surface of the lightguide plate. Furthermore, as now the insulating layer on the surface ofthe first electrodes facing the movable element can be omitted, no lightis absorbed in the insulating layer.

The layer of e.g. 2 micrometer thickness, may contain solid material,e.g. a silicon resin with a refractive index of about 1.4, which issmaller than the refractive index of about 1.5 for a light guide of e.g.ordinary glass or kwartz. However, if the layer comprises a gaseouslayer or vacuum, it is e.g. an air gap. A vacuum gap is also possible.Then the first electrodes are free from contact with the light guide.The first electrodes can be provided by metal wires or strips runningbeside one another. However, if the first electrodes are carried by athird plate, the first electrodes are at a fixed position at the thirdplate. If desired, this third plate can be aligned with respect to thelight guide plate.In an embodiment a surface of the movable element facing the secondplate is provided with an insulating layer, obviating the dielectriclayer on the second electrodes at the second plate.

These and other aspects of the invention will be further elucidated anddescribed with reference to the drawings, in which:

FIG. 1 shows schematically a cross sectional view of the display panel,

FIG. 2 shows schematically a cross sectional view of the display panel,

FIG. 3 shows schematically a portion of the display panel,

FIG. 4 shows schematically a portion of the display panel, and

FIG. 5 shows schematically a portion of the display panel.

The Figures are schematic and not drawn to scale and in all the figuressame reference numerals refer to corresponding parts.

In FIG. 1 the display panel 21 comprises a light guide 2, a movableelement 3, a second plate 4 and a layer 51. First electrodes 5 arearranged on the side of the layer 51 facing away from the light guide 2.The layer 51 has a refractive index smaller than the refractive index ofthe light guide. Second electrodes 6 are arranged on the second plate 4facing the movable element 3. A covering element 7 is connected to thelight guide 2, thus forming a space 8. The display panel 21 furthercomprises a light source 9. Light generated by the light source 9 iscoupled into the light guide 2.

In FIG. 2 the layer 51 comprises a gaseous layer, e.g. an air gap. Thedisplay panel 21 further comprises a third plate 50. The firstelectrodes 5 are arranged on the side of the third plate 50 facing thelight guide. The third plate 50 can be omitted if for instance the firstelectrodes 5 are provided by metal wires or strips running beside oneanother. The light, coupled into the light guide 2, travels inside thelight guide 2 and, due to internal reflection, cannot escape from thelight guide 2 unless the situation as shown in FIG. 3 occurs. FIG. 3shows the movable element 3 locally lying against the light guide 2. Inthis state, part of the light enters the movable element 3. The movableelement 3 scatters the light, so that it leaves the display panel 21. InFIG. 3, this is indicated by means of straight arrows.

In FIG. 4 the movable element 3 is positioned between the light guide 2and the second plate 4 by means of spacers 12 and 13. A third electrode25 is part of the movable element 3. The third electrode 25 can bepresent on the side of the movable element 3 facing the light guide 2,on the side of the movable element 3 facing the second plate 4 or evenbe part of the bulk of the movable element 3. By the application ofvoltages to the first and second electrodes 5,6 and the third electrode25 on the movable element 3, an electric force F is generated whichpresses the movable element 3 against the light guide 2. The contactbetween the movable element 3 and the light guide 2 causes light toleave the light guide 2 and enter the movable element 3 at the locationof the contact. In the movable element 3, the light is scattered andleaves the display panel 21. In FIG. 4 the third electrode 25 is presentat the surface of the movable element 3 facing the second plate 4.Therefore, the second electrodes 6 are covered by an insulating layer 11in order to preclude direct electric contact between the third electrode25 and the second electrodes 6. The insulating layer 11 is not necessaryif the movable element 3 is insulating at the surface facing the secondplate 4. The third electrode 25 is then for instance embedded in themovable element, as shown in FIG. 5, or at the surface of the movableelement 3 facing the light guide plate 2.

1. A display panel comprising a light guide, having a first refractiveindex, a second plate facing the light guide, first electrodes andsecond electrodes, associated with the light guide and the second plate,respectively, and a movable element provided with a third electrode,between the light guide and the second plate, for locally decouplinglight out of the light guide, the electrodes being present for locallybringing the movable element into contact with the light guide by theapplication of voltages to the electrodes, characterized in that betweenthe light guide and the first electrodes a layer is present with asecond refractive index being smaller than the first refractive index,and the light guide is positioned between the first electrodes and themovable element.
 2. A display panel as claimed in claim 1 characterizedin that the layer comprises a gaseous layer or vacuum.
 3. A displaypanel as claimed in claim 2 characterized in that the first electrodesare carried by a third plate.
 4. A display panel as claimed in claim 1characterized in that a surface of the movable element facing the secondplate is provided with an insulating layer.